The 1:1 complex of cytosine and 5-fluorouracil monohydrate revisited

The monohydrated molecular adduct cytosine-5-fluorouracil-water (1/1/1) (denoted CytFur) [systematic name: 4-aminopyrimidin-2(1H)-one-5-fluoropyrimidine-2,4(1H,3H)-dione-water (1/1/1)], C(4)H(5)N(3)O x C(4)H(3)FN(2)O(2) x H(2)O, was determined some 40 years ago [Voet & Rich (1969). J. Am. Chem. Soc. 91, 3069-3075] and is widely cited as the first example of an intermolecular complex between two pyrimidinic nucleobases. In view of the importance of these base associations, CytFur has been reinvestigated with modern laboratory equipment to higher precision and with the location and free refinement of the H atoms. The new experiment reaffirms the results of the original and clarifies the tautomeric form exhibited by the compounds. The asymmetric unit comprises a hydrogen-bonded adduct of the canonical amino-oxo tautomers in an exact 1:1 ratio and a water molecule of crystallization. This cyclic dimer forms a layered structure approximately parallel to the bc plane by joining through hydrogen bonds other such cyclic dimers. Disordered water molecules run through tunnels formed by surrounding molecular adducts along the a axis.